Semiconductor devices, such as transistors, integrated circuits or the like are normally fabricated by forming a multitude of circuits by selectively doping areas in a semiconductor wafer. Once formed, the wafer is particulated into a multitude of discrete devices or chips. From an economic standpoint, it is of the utmost importance to obtain the highest possible yield of acceptable devices from each wafer. Such high yields increase productivity and decrease the unit cost of each device and avoids expenditures on costly capital equipment. The number of acceptable devices on a wafer is particularly affected by the number of contaminating impurities (e.g., Cu, Ni, Fe, Au) on the wafers generated during the semiconductor growing and/or wafer processing steps.
To improve device yields, a number of different gettering techniques have been used in the prior art. Gettering, as it relates to semiconductor processing, is a technique whereby the crystal lattice of the semiconductor material is altered or damaged and the wafer subsequently heated. The heating of the wafer causes contaminating impurities to migrate towards, and be trapped in, the damaged areas. The damage can be induced by a variety of techniques such as mechanical abrading, ion implantation, electron or proton irradiation, or by diffusion of very high doses of impurities, such as phosphorus, into the wafer, to produce stress in the crystal with resultant high density of dislocations.
One such technique is disclosed in U.S. Pat. No. 4,131,487 to Pearce et al. which is assigned to the instant assignee. The Pearce patent discloses the gettering of contaminating impurities to the non-device side of a semiconductor wafer by raster scanning a laser beam thereat to produce lattice damage and strain therein. The wafer is then heated to produce a dislocation array adjacent the damaged regions to relieve the strain and attract contaminating impurities in the wafer away from the active side of the wafter. Additionally, Pearce et al. indicate that the laser beam may be directed at the active side of the wafer so long as the beam avoids those portions of that surface where the devices are to be formed.
Such a gettering technique has been most successful, in particular where the semiconductor devices are formed without damaging the crystal lattice by doping the surface thereof with relatively low concentration of impurities and heating the wafer to diffuse the dopant into the wafer. However, when the devices are formed by ion implantation, a very high concentration of diffused dopants or similar techniques, the device areas become damaged. To repair this damage, it has been a standard practice to heat the wafer in a furnace for an extended period of time at an elevated temperature. Since these techniques produce considerable lattice damage in the device areas, a substantial amount of undesirable impurities may be trapped in such regions during the heat treatment and will not migrate to the remote damaged areas resulting in reduced product yield.
Accordingly, there is a need for a technique for gettering contaminating impurities from device areas to areas remote therefrom where the device areas are dammaged during the formation thereof.